Boiler drop plug



Patented Mar. 9, 1943 BOILER naor- PLUG George McCormick, Redwood City, and Frank E. Russell, San Francisco, Calif.

Application September 7, 1942, Serial No. 457,628

6 Claims.

The present invention relates to safety plugs for steam boilers, of the type wherein a drop plug member is released by the melting of a cementing body of fusible metal, when the temperature of the boiler exceeds a predetermined value, thereby permitting the escape of steam which either smothers the fire or acts as a warning to the engineer.

The general objects of the present invention are the same as those of our prior Patent No. 1,900,- 357, March 7, 1933, viz: to improve the reliability and accuracy of operation of the plug. Our present improvements have for more particular objects the prevention of intercrystalline oxidation of the fusible metal, which might adversely affect its melting and cause the plug to stick, and the provision of a plug member of such shape that it is more easily handled and inserted in the body member during the assembling process, that it is lighter and therefore requires less metal, and that it conducts heat more rapidly to the contact surfaces of the fusible metal, thereby promoting acwhich should be read with the understanding that changes, within the scope of the appended claims, may be made in the form, construction and arrangement of the several parts without departing from the spirit of the invention.

Reference will be made to the accompanying drawing, wherein:

Fig. 1 is a longitudinal section of a preferred form of our plug.

Fig, 2 is an elevation of the drop plug member removed from the body. 7

Fig. 3' is an elevation of a drop of slightly modified form.

The function and mounting of boiler drop plugs are well known in the art and need not be repeated here. Sufi'ice it'to say that accuracy of operation, at a certain predetermined temperature, or within narrow limits on either side thereof, is of prime importance. If the plug lets go at too low a temperature, the boiler must he needlessly withdrawn from service, whereas if the plug sticks or lets go at too high a temperature, disaster may result.

The fusible metal which normally holds the plug member sealed in the body has a tendency to deteriorate with continued or repeated subjection to temperatures slightly below its fusion point. This deterioration in some instances takes the form of intercrystalline oxidation of the fusplug member of the joint, and may result in sticking of the plug member when the fusingtemperature is reached. Tests and experience have shown that such trouble is accentuated when the layer of fusible metal forming the joint is very thin, and

is minimized if not always prevented by increasing the thickness of the layer of fusible metal. On the other hand, a thick layer or body of fusible metal is more subject to creeping and progressive weakening of its holding power than a thin layer.

Another consideration promoting accuracy of operation is rapid and even distribution of heat to all parts of the fusible metal joint, so that, in the event of a sudden increase in temperature above the danger point, all portions of the fusible metal will reach fusion temperature at the same time, and complete collapse will result.

Referring to the drawing, Fig. 1 illustrates a plug designed for use in the crown sheet 6 of a locomotive boiler. The space above the crown sheet 6 is the interior of the boiler, and is normally filled with water to a safe height above the plug. The space below the crown sheet is the interior of the fire box.

The body I of our plug is of usual construction, having an exterior tapered thread 8 by which it is held in a threaded hole in the crown,

" terior is tapered inwardly as atIlI for a distance well above the end of the exterior threads. At the upper end of the tapered portionis a hori-.v

zontal shoulder 12, and above said shoulder is a cylindrical bore l3 extending to the top of the body.

The drop plug member has a shank It and a head I5. The shank fits within the cylindrical bore l3 of the body and the head fits against the shoulder l2 of said body. The shank i4 is of the same length as the bore i3 in which it fits, so

smaller diameter than its lower end where it joinsthe head. The head l5 forms a horizontal shoulder 20 abutting against the shoulder l2 of the body, and the outer periphery of said head is chamfered as at [1, so that its greatest diameter, which is slightly less than the interior diameter ible metal, which begins at the exposed surfaces of the body, is near its shoulder 20.

The body of fusible metal that forms the joint or seal lies between adjacent surfaces of the plug member and the body. The upper portion of said fusible seal above the shoulders l2 and 2D, is a conical annulus I8, between the tapered shank 14 of the plug and the cylindrical bore l3 of the body. This portion of the fusible seal has its greatest thickness at the top I9, where it is exposed to the boiler water and steam.

The lower portion 2| of the seal, below the shoulders l2 and 20, and between the periphery of the head l5 of the plug and the interior of the body, again increases in thickness toward its lower end which is exposed to the fire. The surface area of said lower end is still further increased by the concave meniscus 22 formed by surface tension in the molten metal when the seal is applied. There is preferably no fusible metal between the adjacent horizontal shoulders i2 and of the body and the plug.

It will be seen that the fusible seal has its greatest thickness at the bottom, where it is normally exposed to the highest temperature and the greatest oxidizing effect. Its thickness then decreases to a minimum in its central portion, where it is protected from contact with air,

' gas, or-steam, and finally increases again at the top, where it is exposed to the boiler water and steam. As a result of this construction intercrystalline oxidation of the fusible seal is held to a minimum, by the increased thickness at the ends Where it is exposed to the greatest oxidizing effect, while at the same time the central protected portion of the seal is thin enough to obviate creeping and to prevent loss of holding power. The increased thickness of the fusible metal at both exposed ends also insures formation of a perfect seal at these points. The tapered shank Id of the plug member has an additional advantage in that it facilitates insertion in the bore i3 of the body when assembling.

The plug member is formed with a recess 23 extending up from its bottom and so proportioned widely at their inner ends than at their outer that its walls are about the same thickness throughout. This has three objects, viz: to promote rapid and uniform transfer of heat to the fusible seal, to save metal, and to provide a socket for the insertion of suitable tongs for holding the plug member during assembly.

Fig. 2 shows the plug member as described above, before assembly. Fig. 3 shows a slight modification thereof, designed to simplify manufacturing, in which the shank I4 is cylindrical at its lower portion, as at 24, and tapered or conical at its upper portion, as at 25. The diameter of the upper end is the same as that of the plug shown in Fig. 1, so that the'only difference is that the fusible seal used with the plug of Fig. 3 will have more of its length, in its central region, of minimum thickness. It will still have increased thickness at its exposed ends, as in Fig. l, with all the advantages thereof.

We claim:

.1. A boiler drop plug comprising a body member having a passage through it, one portion in the length of said passage being a cylindrical bore, means for holding said body member in thev wall of a boiler, a drop plug member having a tapered shank fitted within said bore, said shank having its outer end of larger diameter than its inner end, and a body of fusible metal forming a seal between said bore and said shank, said seal ends, and a body of fusible metal filling the space between said adjacent surfaces and forming a seal therebetween, said seal having greater thickness at its inner end than at its outer end.

3. A boiler drop plug comprising a body member having a bore extending through it and an interior shoulder, the portion of said bore lying inwardly from said shoulder being of smaller diameter than the portion lying outwardly therefrom, means for holding said body member in the wall of a boiler, a drop plug member having a head seated adjacent said shoulder and a shank fitted within the smaller portion of said bore, the adjacent surfaces of said shank and said smaller bore portion being spaced more widely at their inner ends than at their outer ends, and a body of fusible metal forming a seal between said adjacent surfaces and between said head and said shoulder, said seal having greater thickness at its inner end than at its outer end.

4. A boiler drop plug comprising a body member having a bore extending through it, said bore having an inner end portion of smaller diameter and an outer end portion of larger diameter, said portions being separated by a shoulder, means for holding said body member in the wall of a boiler, a drop plug member fitted within said bore, said plug member having an inner end portion of smaller diameter, an outer end portion of larger diameter, and a shoulder between said portions, the adjacent surfaces of said bore and said plug member being spaced apart more Widely at the ends of said plug member than at the region of said shoulders, and a body of fusible metal forming a seal between the adjacent surfaces of said bore and said plug member, said seal increasing in thickness from said shoulders toward both ends.

5. A boiler drop plug comprising a body member having a bore extending through it, means for holding said body member in the wall of a boiler, 43, drop plug member seated within said bore, the adjacent surfaces of said plug member and said bore being spaced apart more widely at the ends of said plug member than at its central region intermediate said ends, and a body of fusible metal forming a seal between said adjacent surfaces, said seal being thicker at its ends than at its central region.

6. A boiler drop plug comprising a body member lraving a bore extending throughitQmeans for holding said body member in the wall of a boiler, a drop plug member seated within said bore, the adjacent surfaces of said plug member and said bore being spaced apart more widely at the ends of said plug member than at its central region intermediate said ends, said plug member having a recess extending into it from its outer end, and a body of fusible metal forming a seal between said adjacent surfaces, said seal being thicker at its ends than at its central region.

GEORGE MCCORMICK. FRANK E. RUSSELL. 

